Modified method for analysis of C 2C 5 hydrocarbons in an aromatic—alkane matrix using an automated thermal desorber

Abstract

An automated thermal desorber, Model ATD-50 (manufactured by Perkin-Elmer), packed with a Tenax-TA cold trap was used in conjunction with a 50 m x 0.22 mm I.D. fused-silica BP-1 capillary column to separate light hydrocarbons entrained in an aromatic—alkane mixture. Injection of samples was achieved using the standard injection port fitted to the ATD-50. Collection of the material at −30°C on the cold trap proceeded volatilisation and desorption of the trapped volatiles across 1 m of a heated fused-silica transfer line and onto the head of the column. The column oven is held at −35°C, where refocussing of the desorbed materials takes place. The oven is held isothermally for 8.5 min at −35°C before commencing a dual ramp temperature programme to fully separate the mixture. Sharp, near symmetrical component peaks are obtained across a 235°C span. Cooling is achieved using bottled liquid carbon dioxide pumped into the rear of the gas chromatograph oven. The method overcomes the need for multiple column systems, heartcutting steps or sample splitting for the analysis of light hydrocarbons and liquid mixtures. An added advantage is that users of ATD-50 systems can use the equipment for both adsorbent tube desorption and conventional hypodermic syringe liquid analysis. The method separates some 32 simple components varying in volatility from methane to 1,2,3-trimethylbenzene in less than 30 min.